1. Field of the Invention
The present invention relates to a trigger switch mounted in a power hand tool such as an electric-powered drill or the like, and more particularly, to a trigger switch that switches a switch mechanism installed inside the power tool case according to the sliding of a control unit provided on the outside of the case.
2. Related Art
Conventionally, as a switch circuit for a trigger switch, there is known, for example, the trigger switch circuit for power tool disclosed in JP-A-11-144545. That is, the trigger switch circuit controls the rotation of a motor using a moving contact that moves in tandem with the retraction of an operating lever, such that, when the operating lever is in an OFF state, a motor brake switch is turned ON, the motor is shorted and the brake activated. When the operating lever is pulled in an ON state, the motor brake switch is turned OFF, a power switch is turned ON, and electric power is supplied to the sliding circuit substrate, the motor and a light-emitting diode (LED). The speed of rotation of the motor increases as the operating lever is pulled further, a short switch is turned ON and the rotation of the motor is maintained at high speed.
However, whenever such a switch circuit turns the power switch and the short switch ON and OFF, the switching element always remains controllable. Therefore, when the power switch and the short switch turn ON and OFF, the switching element also is turned ON and OFF, and thus an electric potential difference arises between the contacts of the power switch or the short switch, generating a spark when the power switch or the short switch is turned ON or OFF, which increases frictional wear on the contacts and in turn shortens the working life of the contacts.
In addition, since the rotation of the motor and the lighting of the LED are carried out simultaneously when the power switch is switched ON, it is necessary to add an auxiliary switch that is separate from and independent of the power switch in order to light the LED before the motor rotates. This addition of a component increases the price of the power hand tool or the like and hinders efforts to make to such tools more compact and thus easier to handle and more easily portable.
Moreover, in an effort to make the trigger switch thinner while retaining good dust-proof protection, there is, for example, the trigger switch disclosed in JP-A-2003-109451. This trigger switch incorporates the trigger mechanism inside a box-like case, projects a sliding shaft for external control of the switching outside the case, and mounts a trigger on the outside tip of the sliding shaft, while forcing the terminals of the control element into small through-holes so as to leave substantially no gap through which dust can enter, thus improving dust-proof protection.
Furthermore, an L-shaped metallic heat slinger with good thermal conductivity is fixedly mounted on the case to form a single unit therewith so as to absorb and radiate the heat generated by the control element. A switching lever fixed at one end about which the switching lever inclines is mounted on top of the case. The switching lever sets the rotation of the motor (forward or reverse) and has a neutral OFF position. In order to prevent the switching lever from being damaged, the switching lever switches to either one side or the other so that a trigger stopper of the trigger does not engage even if the trigger is retracted while the switching lever is in the neutral position. Moreover, furthermore, because of the bouncing that always occurs when the contacts switch ON, a brake contact for stopping the power hand tool motor is provided separately from the seesaw mechanism for preventing contact wear.
However, in such a trigger switch, because the heat slinger is L-shaped, when installed in the confined space of a power hand tool the heat comes to be radiated in a single direction. Consequently, when the temperature rises beyond a certain level, the rise in temperature tends to accelerate. As a result, the temperature of only the space on the heat slinger side rises, imparting an unpleasant feel to the place where the power hand tool is gripped.
In addition, because the sliding shaft for external control of the switching protrudes from the case and the trigger is mounted on the outside tip of the sliding shaft, dust gets inside the switch mechanism from a gap between the sliding shaft and a support member supporting the sliding shaft when the sliding shaft slides, which can cause malfunctions of the switch mechanism.
Furthermore, because the trigger switch is constituted so that the switching lever switches to either one side or the other so that the trigger stopper of the trigger does not engage even if the trigger is retracted while the switching lever is in the neutral position, the trigger cannot be operated when the lever is in the neutral OFF position and thus does not function as the safety mechanism that it is originally intended to be. In addition, the brake contacts are provided separately from the seesaw mechanism, thus increasing the number of parts.